Also remedies places where the video backends and core rely on things
being indirectly included.

Also gets rid of global headers

Approximately three or four times now, the issue of pointers being
in an inconsistent state been an issue in the video backend renderers
with regards to tripping up other developers.

Global (ugh) resources are put into a unique_ptr and will always have a
well-defined state of being - null or not null

It may be a bit weird to see calls to static functions in
VertexManagerBase now, but at least it's easier to see what's going on.

This option has no use any more, neither performance nor driver workaround.

Suggested by degasus.